1. Field of the Invention
The present invention relates to a signal processing apparatus for changing the frequency characteristics of an input signal, e.g.--a video or audio signal.
2. Description of the Prior Art
A video tape recorder for recording and playing back video signals contains a frequency modulation/demodulation circuit in which as white noise is developed in FM transmission lines, the level of noise in a demodulated signal is increased by an increase of frequency, which is known as a triangular noise characteristic. For minimizing such an unwanted noise level increase, a signal processing technique is provided in which an input signal of interest is amplified in its medium and high frequency ranges (or emphasized for increase in the frequency deviation) prior to frequency modulation and then, after demodulation, its medium and high frequency ranges are attenuated in the amplitude (or deemphasized). Another noise reduction technique may also be applied in which during recording non-linear preemphasis is provided for a small amplitude, high frequency component of the signal which is then non-linear deemphasized during playback.
However, the increase of frequency deviation by emphasis effects is restricted to a certain level due to confining of the frequency band in an FM transmission line to a limited range determined by electromagnetic transformation effects and thus, the signal-to-noise (S/N) ratio of a reproduced signal will stay low within a limit. This disadvantage is attributed to not only a video tape recorder but also any other systems for transmission of frequency modulated signals e.g. for satellite broadcasting service.
FIG. 11 illustrates an emphasizing circuit of the prior art, in which a video signal fed to an input terminal 110 is emphasized and transferred to an output terminal 114. The emphasizing circuit shown in FIG. 11 comprises a capacitor 111 (capacitance: c), a resistor 112 (resistance: Rb), and another resistor 113 (resistance: Ra). In operation, a signal denoted by a in FIG. 12 is shifted by the circuit to a waveform denoted by b at the output terminal 114. A common video tape recorder is arranged to record such a waveform a of a video signal shown in FIG. 12 onto a magnetic tape through frequency modulation. In practice, the signal is however frequency modulated with its amplitude being clipped between two levels denoted by S1 and S2 shown in FIG. 12, because the frequency band in an electromagnetic transformation system across an FM transmission line is limited in range. Hence, a resultant frequency demodulated signal will contain errors of waveform distortion. If the emphasizing level is attenuated for averting the clipping action, the S/N ratio in the reproduced signal will be reduced by a corresponding degree.
FIG. 13 shows a non-linear emphasizing circuit of the prior art, in which an input signal fed to an input terminal 115 is filtered by a highpass filter (HPF) 116, consisting of capacitor and a resistor, to a high frequency component signal which is in turn amplitude limited by a limiter 117 and added by an adder circuit 118 to the original signal. A sum signal is then transmitted from an output terminal 119. FIGS. 14(a)-14(d) show different waveforms of the input signal in the non-linear emphasizing circuit of FIG. 13, in which FIG. 14(a) illustrates an input signal at the input terminal 115; FIG. 14(b) illustrates an output of the HPF 116,; FIG. 14(c) illustrates an output of the limiter 117, and FIG. 14(d) illustrates output from the output terminal 119. During playback, the non-linear emphasized signal shown in FIG. 14(d) is non-linearly deemphasized to a waveform as shown in FIG. 14(a). At the time, a small amplitude noise in a high frequency component of the signal developed during record and playback action will be attenuated. In FIG. 14(a)-- 14 (d), the period when the output signal of the HPF 116 is amplitude limited by the limiter 117 is represented by T1. However, a part of the noise in each period T1 will hardly be attenuated through the non-linear deemphasizing action during the playback. As the result, noise components remain intensively at the periods T1 where the waveform exhibits sharp changes between rise and decay, thus causing a reproduced image to be reduced in quality. More specifically, the noise component appears at the rear end of each edge between rise and decay and will thus be more noticeable during reproduction.